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math/big: make nat.setUint64 vet-friendly

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nat.setUint64 is nicely generic.
By assuming 32- or 64-bit words, however,
we can write simpler code,
and eliminate some shifts
in dead code that vet complains about.

Generated code for 64 bit systems is unaltered.
Generated code for 32 bit systems is much better.
For 386, the routine length drops from 325
bytes of code to 271 bytes of code, with fewer loops.

Change-Id: I1bc14c06272dee37a7fcb48d33dd1e621eba945d
Reviewed-on: https://go-review.googlesource.com/38070
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
This commit is contained in:
Josh Bleecher Snyder 2017-03-10 15:09:05 -08:00
parent 95c5227c15
commit 2de773d45f
2 changed files with 5 additions and 17 deletions
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2
src/cmd/vet/all/whitelist/64bit.txt
View File

@ -22,7 +22,5 @@ math/big/arith.go: (xi&yi | (xi|yi)&^zi) might be too small for shift of 63
math/big/arith.go: (yi&^xi | (yi|^xi)&zi) might be too small for shift of 63
math/big/arith.go: xi &^ zi might be too small for shift of 63
math/big/arith.go: (zi &^ xi) might be too small for shift of 63
math/big/nat.go: t too small for shift of 64
math/big/nat.go: x too small for shift of 64
math/big/nat.go: yi might be too small for shift of 60
math/big/nat.go: yi might be too small for shift of 60

20
src/math/big/nat.go
View File

@ -68,24 +68,14 @@ func (z nat) setWord(x Word) nat {
}
func (z nat) setUint64(x uint64) nat {
// single-digit values
// single-word value
if w := Word(x); uint64(w) == x {
return z.setWord(w)
}
// compute number of words n required to represent x
n := 0
for t := x; t > 0; t >>= _W {
n++
}
// split x into n words
z = z.make(n)
for i := range z {
z[i] = Word(x & _M)
x >>= _W
}
// 2-word value
z = z.make(2)
z[1] = Word(x >> 32)
z[0] = Word(x)
return z
}